A material where, for example, a DLC (Diamond like Carbon) or a ceramic hard film is coated on the surface of a metal member can have improved properties such as abrasion resistance or slidability. Therefore, the material is applied to a member used in a severe environment. Among them, a sliding member coated with a DLC film has been widely used since it is excellent in abrasion resistance and slidability.
However, when a DLC film, for example, is formed on an alloy to be surface-coated through a P-CVD (plasma CVD) or PVD process, there is a problem in adhesion. In order to improve adhesive strength, the substrate needs to be heated to a high temperature during depositing the film. That is, the alloy to be surface-coated is heated to a high temperature of 400 to 500° C. If the material softens during heating, the DLC film will be easily released. Therefore, it is necessary to select an alloy to be surface-coated which is hardly softened even at a high temperature of 500° C.
Therefore, a high speed tool steel which is not softened to 58HRC or less even at high temperatures is preferably used as an alloy on which a DLC film is coated using the P-CVD (plasma CVD) or PVD process (see JP-A-2003-27236). The high speed tool steel contains a large amount of alloying element such as Mo, W, V, or Nb. Due to these alloying elements, the steel can maintain its hardness at a high value even during forming a DLC film through the P-CVD (plasma CVD) or PVD process, for example.    [PATENT DOCUMENT 1] JP-A-2003-27236